A composite video signal of NTSC system is composed by superposing a chrominance signal on a luminance signal in an interleaving relation. It hence requires a process of separating the two signals, but if a chrominance signal component is mixed in the separated luminance signal, it appears as dot interference (caused by chrominance signal) on the screen, or if a luminance signal component is mixed in the chrominance signal, it results in cross color interference, and it is therefore ideal to separate them without causing such mixing. As a method of separating, recently, it is the mainstream to separate by a comb filter using a 1H (H being a horizontal scanning period) delay circuit.
FIG. 6 is a schematic diagram showing an example of signal processing circuit for separating a composite video signal of a conventional NTSC color television system into luminance signal and chrominance signal (hereinafter called Y/C separation). In FIG. 6, a video signal 701 is entered, and is fed into a 1H delay circuit CCD HDL 702 of CCD (charge coupled device), and is delayed by a horizontal scanning period, and the 1H delayed signal is delivered from the 1H delay circuit 702 into a subtractor 703. In the subtractor 703, the input video signal 701 is also supplied, it is subtracted in relation to the 1H delayed signal. As known well, since the chrominance signal is inverted in phase, according to the principle of interleaving, in every adjacent scanning line, and therefore if the video signal has correlation between lines, the luminance signals are mutually canceled by the subtraction process, and the component of chrominance signal is separated as the output signal of the subtractor 703.
On the other hand, the input video signal 701 is also supplied into a subtractor 704. The subtractor 704 is also provided with a chrominance signal obtained by passing the output signal of the subtractor 703 through a band pass filter circuit BPF 705, and therefore by subtracting this chrominance signal from the video signal 701, only the luminance signal is separated and obtained.
This operating function is the principle of separation of luminance signal and chrominance signal by the known comb filter. However, since perfect separation is impossible in the position not having correlation between signals (between lines) before and after 1H, dot interference or cross color interference may occur. Accordingly, by detecting a signal not separated in a comb filter circuit, it is attempted to process so as to decrease such interference.
That is, in the prior art in FIG. 6, the output signal of the subtractor 703 is supplied into a low pass filter LPF 711 and a band pass filter BPF 713, and a luminance signal component of lower frequency than the chrominance signal component is taken out from the LPF 711, and a chrominance signal component is taken out from the BPF 713. If there is no correlation in signal between lines, the luminance signal component is issued through the LPF 711, and is put into a YL correlation detecting circuit 712, and a signal showing absence of correlation is issued from the YL correlation detecting circuit 712.
On the other hand, if the entered video signal has a chrominance signal, a chrominance signal component is issued from the BPF 713, and is put into a chrominance detecting circuit 714. This chrominance detecting circuit, if the chrominance signal component is entered, issues a signal showing its detection.
Between thus obtained no-correlation detection signal and the detection signal of chrominance signal, the AND (logical operation) is calculated in an AND circuit 715. By the output signal of the AND circuit 715, switch operations of trap circuit 707 and switch circuit 706 are controlled.
That is, if there is no correlation in signal between lines and a chrominance signal is present, a switch 708 of the trap circuit is closed, and the chrominance signal component superposed on the luminance signal is removed by the trap effect, so that the dot interference is lessened. At this time, the switch circuit 706 operates so that a chrominance signal may be directly obtained from the video input signal 701.
However, since the chrominance signal is detected directly from the signal obtained from the output signal of the subtractor 703 in FIG. 6 through the BPF 713, if there is no correlation between lines, a luminance signal component is present in the output signal of the subtractor 703, and hence the precision of chrominance signal detection is lowered. Accordingly, depending on the pattern of the image, the Y/C separation may not be realized at high precision.
To enhance the precision further, in a prior art, for example, the luminance signal obtained in the comb filter circuit is passed again through other comb filter circuit, and presence or absence of chrominance signal component contained in the luminance signal is detected (not shown in a drawing). In this case, however, one more 1H delay circuit for color video signal is needed, and the cost is increased.
If there is no correlation in signal between lines, as mentioned above, since a luminance signal component is contained in the chrominance signal obtained from the output signal of the subtractor 703 in FIG. 6, it is passed through the band pass filter circuit BPF 705, and the cross color interference is lessened. Although such filter circuit can lessen the cross color interference in the narrower band characteristic, the transient characteristic of chrominance signal is impaired, and the response of chrominance signal is sacrificed in the signal high in correlation between lines and free from cross color interference.